From the character builder, click Load to bring up all of your saved characters, select the one you want, then click export to bring up a save dialog.Here's my raw output from the chargen...I don't know how to get the actual saved file onto this site though. Happy to do so if someone can tell me how.
Below the 'submit reply' and 'preview post' buttons on a new post (or 'edit advanced', or just below the post content on 'edit', there's a 'manage attachments' button. Click it to pop up an edit window (you may need to resize it). In the 'upload file from your computer' box, use one of the 'Browse...' file upload buttons to select the .dnd4e file, then click on the 'upload' button in the 'upload file from your computer section'. You'll see a 'please wait' message under 'Upload File from a URL' for a bit (depending on connection upload speed) then the popup window will refresh. When that's done, scroll down and click on 'close this window'. Then submit your new post or save your edit normally.